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pythonoopargumentsintrospection

Introspecting arguments from the constructor function __init__ in Python

What is a way to extract arguments from __init__ without creating new instance. The code example:

class Super:
    def __init__(self, name):
        self.name = name

I am looking something like Super.__dict__.keys()type solution. Just to retrieve name argument information without adding any values. Is there such an option to do that?

Solution

  • Update for Python 3.3+ (as pointed out by beeb in the comments)

    You can use inspect.signature introduced in Python 3.3:

    class Super:
    def __init__(self, name, kwarg='default'):
        print('instantiated')
        self.name = name

>>> import inspect
>>> inspect.signature(Super.__init__)
<Signature (self, name, kwarg='default')>

    Original answer below

    You can use inspect

    >>> import inspect
>>> inspect.getargspec(Super.__init__)
ArgSpec(args=['self', 'name'], varargs=None, keywords=None, defaults=None)
>>>

    Edit: inspect.getargspec doesn't actually create an instance of Super, see below:

    import inspect

class Super:
    def __init__(self, name):
        print 'instantiated'
        self.name = name

print inspect.getargspec(Super.__init__)

    This outputs:

    ### Run test.a ###
ArgSpec(args=['self', 'name'], varargs=None, keywords=None, defaults=None)
>>>

    Note that instantiated never got printed.